Carbon transmitter with moving electrode fins for granule loosening and enhanced electrode coupling



April 1969 M. c. HUFFSTUTLER. JR, ET AL 3,437,76

CARBON TRANSMITTER WITH MOVING ELECTRODE FINS FORGRANULE LOOSENING AND ENHANCED ELECTRQDE COUPLING 'Filed Dec. 20, 1965 IHIIIIIIIIIIIIIIIIID,"

M. c. HUFFS TUTLER JR. INVENTORSBT. KERNS 7 A TTORNEV United States Patent O 3,437,764 CARBON TRANSMITTER WITH MOVING ELEC- TRODE FINS FOR GRANULE LOOSENING AND ENHANCED ELECTRODE COUPLING Miles 'C. Hutfstutler, Jr., Berkeley Heights, and Becky T. Kerns, Summit, N.J., assignors to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed Dec. 20, 1965, Ser. No. 515,059 Int. Cl. H04r 21/02 U.S. Cl. 179-124 9 Claims ABSTRACT OF THE DISCLOSURE A study of granule packing in a carbon transmitter led to the identification of a region in the carbon aggregate which accounts for most of the resistance modulation. To assure that this region stays freest of packing, several fins are attached to the moving electrode and extended into the region. As the diaphragm is vibrated, the fins slice in and out of the region loosening the granules and promoting shear coupling particularly in that region.

This invention relates to an improved granular carbon telephone transmitter and more particularly to a novel geometry for the carbon chamber of such a transmitter that alleviates in some measure the problem of carbon granule packing.

The occurrence of packing in the carbon aggregate in telephone transmitters is a well-known irregularity in the telephone plant. Its causes have been attributed variously to type of coal, size and shape of the granules, shape of the carbon chamber and other sources. Its effect is a significant and serious reduction in the voltage-modulating ability of the aggregate, resulting in a weak or distorted transmission. These effects are especially pronounced for transmitters housed in vertically suspended handsets, as in wall telephone units and coin telephones.

The basic carbon granule transmitter in use today employs a fixed electrode and a moving electrode, between which is held the granular carbon aggregate. A diaphragm to which the moving electrode is attached vibrates in response to acoustic signals impinging on it. The vibratory movement is imparted to the dome electrode which in turn agitates the carbon aggregate. The resistance of the aggregate is varied thereby, and accordingly the voltage drop between electrodes is modulated.

For a given acoustic pressure, this modulation is at a maximum when the aggregate is in a loosely packed condition. This desirable condition is promoted principally by physical or mechanical agitation of the aggregate. The chief sources of this are: acoustic signals of suflicient amplitude;'and the impact incident to replacing the receiver on the switchhook. Frequently, however, these forces are not sufliciently strong or recurrent to produce and maintain the desired unpacking. Acoustic power of speech, for example, varies considerably from person to person. Moreover, many people replace the handset in its cradle with great care specifically to avoid jarring the instrument.

Previous investigations of fundamental granular carbon transmitter problems, including carbon packing studies, appear to rely implicitly upon the classical premise that voltage, or resistance, modulation in the aggregate results essentially from elastic distortion of the granules during mechanical compression and rarefaction of the aggregate. This distortion, it is held, varies the surface resistance of each granule which in turn modulates the D-C voltage applied between electrodes.

The above explanation, however, does not account satisfactorily for certain observed phenomena. For example,

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our study of resistance modulation in a packed or highbulk-density granular aggregate indicates that, with respect to the D-C resistance in a quiet interval, the modulation is asymmetrical, being predominantly in the direction of an increase of resistance rather than the symmetric increase/decrease which occurs in the loose" state. The classical modulation theory does not anticipate this asymmetrical modulation that occurs in the packed state, which suggests the presence of other processes, or perhaps different processes altogether. Our work in this area supports, we believe, a more satisfactory explanation of resistance modulation in carbon transmitters, the principles of which are applied in accordance with our invention to alleviate the packing problem.

More specifically it may be shown that, when the electrodes of the above-described general type carbon transmitter are masked in various ways to confine current flow to selected portions of the aggregate volume, certain regions of the electrode are appreciably more sensitive than others. In particular, the aggregate volume between the barrel portion of the moving electrode and the opposite region of the fixed electrode accounts for a disproportionately large amount of the resistance modulation. This region of optimal coupling is characterized principally by the fact that the granules therein are subject to the greatest shear displacement. That is, the agitation of the carbon in this segment of the chamber results primarily or solely from frictional forces generated by the moving electrode, which operate tangentially to the abutting segment.

The implication of the above are, first, that a substantial fraction of the resistance modulation is associated with shear agitation adjacent the dome of the moving electrode; and, secondly, that by enhancing this shear agitation, the carbon packing problem necessarily is lessened.

Accordingly, a general object of the invention is to improve the performance of the carbon granule telephone transmitter.

Another object of the invention is to reduce substantially the tendency of the carbon granules in such transmitters to become packed and thereby lose their currentmodulating abilities.

A further object of the invention is to reduce the adverse elfects upon the remainder of the transmission plant of granule packing in carbon transmitters.

These and other objects are achieved in accordance with the above inventive precepts, broadly, by maintaining the granules in the optimal coupling region in a loose state by the addition of a plurality of fins attached to the dome electrode and extending into that region.

In accordance with one aspect of the invention, each fin is disposed in substantially normal relation to the surface of the moving electrode .to which it is attached, and extends substantially into the optimal coupling region. In response to normal movement of the electrode, each fin slices in and out of this region and promotes the desired shear coupling. Pursuant to the invention, the fin movement not only tends to loosen up the carbon granules if they are packed but also promotes the type of coupling that has been found most conductive to good resistance modulation.

In accordance with a further aspect of the invention, the current flow between a fixed electrode and a moving dome-shaped electrode may be localized in an annular volume of carbon situated between the barrel of the dome and the opposite conductor band on the back electrode. This arrangement directs current flow through those regions of the carbon which are optimal- 1y coupled to dome movements.

A prime feature of the invention, therefore, resides in a moving electrode with fin extensions or other protuberances that provide an increased contact area, in

the mechanical and electrical sense, with the active portions of the granular carbon.

A related feature of the invention resides in the inclusion of fin extensions on the moving electrode of a carbon transmitter which extend into the region of optimal coupling and that effect therein a loosening of the aggregate with a minimum of acoustic energy.

These and other objects and features of the invention will be more readily apprehended from the detailed description to follow of an illustrative embodiment thereof and from the drawing in which:

FIG. 1A is a sectional view of a carbon transmitter embodying the inventive concept;

FIG. 1B is a schematic detail showing mounting of the moving elctrode;

FIG. 2 is a side view of the transmitter;

FIG. 3 is a schematic diagram of a carbon chamber and dome electrode;

FIG. 4 is a schematic diagram of a dome electrode equipped with shear fins; and

FIG. 5 is a schematic diagram of an illustrative type of fin.

As shown in FIG. 1A, the transmitter unit, designated generally as 10, comprises a circular metallic frame 11 with a shallow cylindrical seat 12, an annular inwardly extending flange 13, an annular .seat 14 and an annular outwardly directed flange 15 coaxial with both seats 12 and 14.

A diaphragm 16, advantageously formed in one piece of a selected duplex sheet material having defined thermomechanical behavior comprises a flat, circular peripheral portion 17, an adjacent annular raised corrugation 18 and a central frusto-conical portion 19. The peripheral portion 17 seats upon the ring-shaped end of seat 12 on the frame 11, bringing corrugation 18 adjacent seat 12. corrugation 18 is flexible to allow piston-like movement of frusto-conical portion 19.

Directly in front of the diaphragm 16 is stretched a compliant moisture-proof membrane 20 covered by a multi-apertured protective grid or cap 21. Membrane 20, grid 21, peripheral portion 17 of diaphragm 16 and a plastic washer 22 are secured together against the ring end of seat 12 by a ferrule 23 crimped over frame 11 at one end and over washer 22 at the other end.

The upper portion of diaphragm 16 terminates in a circular shoulder 24 providing a mounting for a moving electrode 25 that is vibratile therewith. As shown in 1B, a pair of circular opposed outwardly extending shoulders 26 disposed on the base of electrode 25 crimp together to grip shoulder 24 of diaphragm 16, two spacers 27, 28 formed of copper and the lower shoulder 29 of a cup-shaped flexible insulative washer 30.

Advantageously, diaphragm 16 may be fabricated as a bimetallic stamping so that the terminal gradients produced by normal telephone operation are compensated by diaphragm flexure rather than thermo-mechanical displacement of the moving electrode.

FIG. 1A also shows a circular metallic fixed electrode 31 coaxially situated above moving electrode 25. Beetrode 31 comprises an outwardly extending flange 32, an upwardly extending flange 34 and a truncated conical interior surface 35. Another insulative cup 36 seats be tween the inner portion of flange 15 and the outer portion of flange 32. Cup 36 extends beneath the lower surface of flange 32 and rests upon the upper shoulder 33 of insulative member 30.

Electrode 31 is held firmly in this position by an annular plastic washer 37 that is forced against flange 32 by one flange 38 of a metal ring spring 39, the opposite flange 40 of which is crimped under flange 15 in several places as shown at 41 in FIG. 2. A deep cupshaped metallic cap 42 with a grooved inwardly extending leg portion 43 is secured to the upper flange 34 of electrode 31 by a gripping cap 53. Electrical connections (not shown) are made to ring 39 and cap 53 to provide a D-C bias level of about 3 volts.

The space or volume bounded by interior surface 35, flexiber washer 39, the exterior surface of electrode 25 and the interior surface of cap 42 defines the carbon chamber. Filling this chamber is an aggregate of carbon granules designated as 60, filling the chamber to a level indicated at 61, leaving the volume beneath the inner dome of cap 42 empty. A plurality of fins 49 are disposed on electrode 25 in a manner, and for reasons, shortly to be described.

As earlier noted, a region of optimal coupling has been found to exist between the fixed and moving electrodes. FIG. 3, representing a typical prior art electrode and carbon chamber geometry, depicts by solid lines this optimal coupling region. As shown, a moving domeshaped electrode 44 juxtaposed to a fixed electrode 45 defines a chamber 46 for carbon granules. Optimal coupling occurs in a volume region between a moderately narrow annular band 47 on the dome electrode, and the adjacent narrow conical subsection 48 of the fixed electrode. The carbon granules adjacent this barrel reigon of the dome or moving electrode are agitated only from tangential friction, i.e., the moving electrode displacement produces a relative motion, essentially a shear displacement between adjacent layers of granules. This region accounts for the greatest amount of modulation in proportion to its size: the region 47 represents approximately 3040 percent of the total area, but accounts for about 80 percent of the total modulation.

This barrel region has been found to be characterized by its ability to impart to the aggregate the greatest degree of shear displacement, that is, agitation of the carbon prompted primarily by the aforementioned tangential friction. Pursuant to an important aspect of the invention, this effect is enhanced by the incorporation onto the moving electrode of a plurality of narrow fins 49, as shown in FIG. 1A, symmetrically disposed around the barrel region and normal to the surface thereof. FIG. 4 shows this configuration in detail. As shown in FIG. 5, each fin 49 comprises advantageously a relatively sharp forward edge 50 which widens out to form a wedgeshaped surface 51. This wedge is supported by two sharply juxtaposed surfaces 54, 55, and moves in the direction of arrow 52, shown in FIG. 4, in response to vibration of electrode 44. The motion of each of the three fin surfaces 51, 54, 55, is primarily one of shear with respect to the granular aggregateparticularly surfaces 54 and 55. The fins promote modulation in the interface region because they increase the amount of tangential force applied to the aggregate and because their wedging movement within the area of optimal coupling helps keep the granules in the desired loose state.

A variation of the inventive concept requiring less acoustic driving energy is depicted in FIG. 1A, in which the moving electrode is conically shaped and the fins 49 are mounted symmetrically around the flat sides of the cone. Four such fins are employed, spaced degrees apart. The cone of the moving electrode 25 and the adjacent truncated conical portion of surface 35 are substantially parallel, element for element. The extent of shear coupling generated by the conically-shaped electrode 25 is inherently somewhat greater than that generated by the dome-shaped electrode 44. Addition of fins 49 to the electrode 25 in accordance with the invention, significantly increases the total shear coupling.

It has been found that the geometry of the carbon granules employed in a chamber designed to maximize shear coupling has an appreciable effect upon the extent of shear coupling actually achieved. Granular aggregates in which the average particle diameter is in the range of 0.006 inch to 0.007 inch and the particles are slightly eccentric (mean axial ratio 1.4 to 1.6) were found to be especially suitable for this type carbon chamber. A maximum of 0.1 percent by weight of iron impurity in the aggregate is desirable for optimum stability in use. It is also important that in the loose condition there be a minimum current dependence of resistance, as, defined by (P =minimum bulk resistivity at 6 X 10- amp/cm.

current density (5 Hz.)

The minimum acceptable value for high sensitivity has been found to be 0.39=E. This can be controlled by selection of raw materials (anthracite coal) and improved roasting methods.

Thermo-mechanical oscillations of the moving electrode system (diaphragm 19 and electrode 25) may lead to loss of sensitivity. To avoid this, the electrode-diaphragm assembly should be constructed in such a way that no thermal expansion occurs. This goal is achieved by use of low expansion alloys such as nickel plus 35 percent iron for both electrode and diaphragm. Alternatively, as earlier mentioned, use of a duplex (bimetallic) diaphragm stamping helps maintain the position of the moving electrode despite thermal gradients in the diaphragm assembly.

The performance characteristics of a carbon transmitter incorporating the present inventive concept compares favorably in most respects to a conventional carbon transmitter, and in addition, exhibits a significant capacity to keep the aggregate in the desired loose state. This comparison is illustrated by the following data:

Measured under identical conditions in a standard test circuit for telephone transmitters.

Various changes and modifications may be made to the invention herein described by persons skilled in the art without departing from the spirit and the scope of the claims to follow.

What is claimed is:

1. A transducer comprising, in combination, a fixed electrode, a moving electrode, an aggregate of carbon granules therebetween, and a plurality of spaced narrow wedgelike fins affixed to said moving electrode and wlth the wedge portion extending into said aggregate in alignment with the direction of movement of said moving electrode, each said fin being responsive to vibratory movement of said moving electrode for agitating said granules in the vicinity of said fins, thereby to loosen said aggregate.

2. A transducer in accordance with claim 1 wherein said fixed electrode comprises a conical surface portion and said moving electrode comprises a congruent conical surface portion in spaced, axial alignment with said fixed electrode, a portion of said aggregate being contained in the volume between said conical surface portions of said fixed electrode and said moving electrode, said aggregate portion defining a coupling region, and said fins extending into said coupling region.

3. A transducer in accordance with claim 2 wherein each said fin further comprises a pair of adjacent convergent surfaces defining a narrow wedge extending normally from said moving electrode into said aggregate, each said fin being responsive to movement of said electrode for imparting a substantial tangential frictional force to said granules in its vicinity, thereby to enhance shear coupling between said fixed and moving electrode conical surface portions.

4. A transducer comprising, in combination, a fixed electrode, a moving electrode, an aggregate of carbon granules therebetween, means for effecting a shear coupling between a selected annular surface region of said moving electrode and an adjacent annular surface region of said fixed electrode, the aggregate volume between said regions defining a coupling zone, and a plurality of spaced fins afiixed to said moving electrode and extending into said coupling zone responsive to vibratory movement of said moving electrode and agitating said granules in said zone.

5. A transducer in accordance with claim 4 wherein each said fin comprises a relatively sharp upper blade and a relatively blunt lower surface, said upper blade being responsive to vibration of said moving electrode for imparting a shear force to said aggregate, thereby to loosen same.

6. A transducer in accordance with claim 5 wherein said carbon aggregate comprises carbon particles having an average diameter in the range of 0.006- inch to 0.007 inch and an average particle eccentricity in the range of 1.4 to 1.6.

7. A transducer in accordance with claim 6 further comprising a vibratile acoustic diaphragm and means for mounting said moving electrode on said diaphragm, said moving electrode and said diaphragm being formed of a low expansion alloy plated with a noble metal.

8. A transducer in accordance with claim 7 wherein said moving electrode and said diaphragm are formed of an alloy comprising nickel plus 35 percent iron and wherein each said fin is formed of a low expansion alloy electroplated at least in part with a noble metal.

9. A transducer in accordance with claim 8 wherein said plurality of fins comprises at least four fins spaced an equal distance apart from one another on said moving electrode and extending into said coupling zone.

References Cited UNITED STATES PATENTS 1,130,029 3/1915 Shreeve 179125 KATHLEEN H. CLA'FPY, Primary Examiner.

ARTHUR A. MCGILL, Assistant Examiner. 

